A communications network comprises a plurality of (that is to say at least two) participants connected to one another via suitable message channels. The message channels may comprise wireless and wired channels. In a communications network, information is usually transmitted from one participant to another in the form of discrete data packets. Such data packets are referred to as messages below.
The problem arises in practice of a message not being correctly transmitted from one participant to the other. There are various possible causes of this. Interference in the transmitting participant (the message is not transmitted or is not correctly transmitted), on the transmission path (for example in the case of brief interference in the message channel) and in the receiving participant (for example brief failure of a receiving stage of the receiving participant) comes into consideration.
It is known practice to increase the reliability of the information transmission in the communications network by repeatedly transmitting the information to be transmitted in succession.
In a first method which can be implemented, for example, in conventional field bus systems such as PROFIBUS or else in PROFINET, a first participant (for example a master participant) transmits a message to a second participant (for example a slave participant) and then waits for a response. For example, provision may be made for the slave participant to respond with a separate message. If the first participant does not receive a response within a defined time, the first participant transmits the originally transmitted message once again.
In a similar second method, the participants process the message in passing (for example in the case of EtherCAT). In that case, a first participant can be operated in such a manner that said participant transmits a message and then waits for the return of the transmitted message. If the first participant does not receive the expected returning message, the first participant transmits the originally transmitted message once again.
One disadvantage of these methods is that the transmitting participant must wait for the reception of a message in an event-controlled manner. This may result in an increased interrupt load, in particular in systems in which communication and application run on a common processor.
In a third method, instead of an individual message, a participant transmits a message repeating sequence which comprises a plurality of messages having substantially the same contents. In order to successfully transmit the useful data (that is to say the information to be transmitted), it then suffices for the receiving participant to receive at least one of the messages from the message repeating sequence. A participant may be set up, for example, in such a manner that said participant evaluates a first successfully received message in the message repeating sequence and ignores, that is to say does not evaluate, all further messages in the message repeating sequence which are possibly received by the participant.
If the participant is set up to initiate a particular action in response to the successful reception of the one message in the message repeating sequence, the time at which the action is performed will depend on which message in the message repeating sequence the participant reacts to in one simple embodiment. If the participant does not react to the first message, but rather to a subsequent message in the message repeating sequence, the participant will initiate the action later than if reacting to the first message in the message repeating sequence. This may be problematic, in particular, when a plurality of participants in the communications network are intended to be operated synchronously with the aid of messages which are matched to one another in terms of time, for example in an automated production plant having a plurality of distributed participants which must synchronously perform particular actions, for example the movement of a component.
The patent specification EP 2 527 935 B1 describes a method for operating an automation system, in which a measure of an offset between an occurrence of a synchronization signal and a communications clock is determined on the transmitter side. The determined measure of the offset is transmitted during data transmission to at least one receiver. After receiving the determined measure, the receiver waits for a corresponding period of time to elapse and generates an output signal in response to the synchronization signal after the period of time has elapsed.